The area near the top ring of a gasoline engine reaches high temperatures of 200° C. or higher due to fuel combustion. In an internal combustion engine, repeated impact occurs between the piston ring and the piston ring groove surface of the piston (hereunder referred to as “ring groove surface”) by combustion pressure under such high temperature, while the piston ring surface and the ring groove surface simultaneously slide in the circumferential direction. Such impact and sliding with the piston ring under high temperature causes fatigue fracture on the ring groove surface and dropping off of protrusions on the surface, exposing a fresh active aluminum alloy surface on the ring groove surface. Also, the fresh aluminum alloy surface exposed on the fallen aluminum alloy strips and in the ring groove contacts with the top side and bottom side of the piston ring impacting with the piston ring, and results in even more sliding. This causes the aluminum alloy strips to adhere onto the sides of the piston ring, creating a state of “aluminum adhesion” in which the piston ring body becomes anchored to the fresh aluminum alloy surface of the piston. The aluminum adhesion continues as long as fresh aluminum alloy surface continues to be exposed, and progressive aluminum adhesion results in fixation of the piston ring onto the piston in the ring groove, which impairs the seal performance of the piston ring. Loss of the gas seal function, which is one of the seal performances, results in a blowby phenomenon in which high-pressure combustion gas bleeds out from the combustion chamber into the crankcase, and this can lead to lower engine output. Also, loss of the oil seal function leads to increased oil consumption. Furthermore, aluminum adhesion results in progressive ring groove wear and loss of the seal property between the top and bottom sides of the piston ring and the ring groove surface, leading to increase in amount of blowby.
Many methods have been proposed in the past to suppress aluminum adhesion, such as methods of avoiding direct contact between the base material of the piston, i.e. the aluminum alloy, and the piston ring, and especially the top ring, and methods of alleviating impact of the piston ring on the ring groove.
As one countermeasure on the piston side, Patent Literature 1 describes a method of performing anode oxidizing treatment (alumite treatment) of the ring groove surface, and filling a lubricating substance into the micropores produced by the treatment. Alumite treatment forms a hard film composed mainly of aluminum oxide on the ring groove surface, and therefore suppresses fall-off of aluminum alloy which is the base material of the piston, and minimizes adhesion onto the piston ring. However, the cost necessary for anode oxidizing treatment of the piston is high, and aluminum oxide, being hard, is associated with the problem of poor initial fitting properties.
On the other hand, as a countermeasure on the piston ring side, Patent Literature 2, for example, describes a method of forming on the piston ring sides, a film comprising molybdenum disulfide or the like, as a solid lubricant, dispersed in polyamide, polyimide or the like as a heat-resistant resin. In the construction described in Patent Literature 2, the solid lubricant in the film undergoes cleavage and wear, the frictional coefficient of the film is lowered, attack on the ring groove is alleviated and aluminum adhesion is minimized.
Also, Patent Literature 3 teaches that forming a polybenzimidazole resin film containing a solid lubricant on the top and bottom sides of the piston ring can effectively suppress the aluminum adhesion phenomenon. Patent Literature 3 also indicates that carbon fibers or glass fibers can also be added instead of a solid lubricant.
With increasingly high output of engines in recent years, higher temperatures are being reached in the area near the top ring. Because of this situation, fatigue fracture is becoming more common due to lower piston strength, and it is becoming difficult to maintain the resin films covering piston rings for long periods. Although a solid lubricant is added as an essential component in Patent Literature 2, the solid lubricant itself undergoes cleavage and wear, as mentioned above, thereby lowering the frictional coefficient of the film and alleviating attack on the ring groove. Consequently, the wear resistance of the film is lower, and it is difficult to maintain the film for long periods and to sustain an effect of suppressing aluminum adhesion. In addition, there are limits to the amount of solid lubricant that may be added to minimize such film wear, and hence limits on the degree to which frictional coefficient of the film can be reduced. Consequently, the surface of the piston material whose hardness has been reduced under high temperature becomes roughened, and this can cause even further aluminum adhesion.
With the film of Patent Literature 3 as well, a solid lubricant is added as an essential component, and therefore the wear resistance of the film is lower, and it is difficult to maintain the film for long periods and to sustain an effect of suppressing aluminum adhesion. Thus, it has also been considered to add carbon fibers or glass fibers for the purpose of increasing the wear resistance and strength of the film. However, ordinary carbon fibers such as polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers and cellulose-based carbon fibers, or glass fibers, have fiber diameters of 5 to 10 μm. When filler with such a large fiber diameter is added, and the ends of the filler protrude through the film surface, this can potentially cause damage and wear on the surface of the ring groove. In addition, when filler with such a fiber diameter is added, the film becomes thickened. However, there are limits to the initial film thickness, since suitable clearance must be provided between the piston ring side and the ring groove, and it is not desirable to thicken the film in order to minimize increase in the blowby or oil consumption that result from increased clearance after attrition of the film.
Under the current circumstances, therefore, it has not been possible to obtain a piston ring that can sustain an excellent effect of suppressing aluminum adhesion over long periods for high-output engines.